1. Field of the Invention
The present invention relates to electronically controlled timepieces, and more particularly to a circuit whose components are simplified so that they may be easily fabricated in a monolithic integrated circuit.
2. Prior Art
Most electronic circuits used in conventional electronic timepieces require several separate electronic components whose fabrication by monolithic integrated circuit techniques is extremely difficult.
In a typical electronic timepiece as shown in the block diagram of FIG. 1, electric signals from an oscillator means are detected by a mechanical-electrical transducer and, after appropriate control and amplifying functions are performed by an electronic circuit, the modified signals are used to supply energy to sustain the vibration of the oscillator by means of an electromechanical transducer. Thus each component is a link in a closed loop. The signal from the oscillator means is relayed and transformed by suitable means to a time display device.
The electronic circuit shown in FIG. 2 is commonly employed in prior art electronic timepieces of the balance wheel, tuning fork and sound fragment types. A phase sensing coil, Ls, is usually part of the electro-mechanical transducer. Transistor Qo and circuit elements, C, R, and parasitic capacitance Cx correspond to an amplifying and control circuit, and drive coil, L.sub.D, is part of the electromechanical transducer. The oscillator means may be a tuning fork having small permanent magnets on its tines or a balance wheel having appropriately disposed magnetic elements. An electromagnetic flux linkage serves to couple the energy between the oscillator means and the transducers. A major reason why the circuit illustrated in FIG. 2 cannot be converted to a monolithic integrated circuit is due to the difficulty in fabricating the resistor and capacitor elements according to integrated circuit techniques. For example, circuit values required for the balance wheel type are 0.47 .mu.F for capacitance and 10 megohms for resistance, and for the tuning fork type 0.22 .mu.F for capacitance and 2.2 megohms for resistance for the circuit illustrated in FIG. 2. The achievement of these capacitance and resistance values using ordinary bipolar fabrication techniques has been considered difficult. However, it has recently been possible (Koehler, U.S. Letters Pat. No. 3,727,151) to fabricate the circuit shown in FIG. 2 according to monolithic integrated circuit techniques for electronic timepieces of the tuning fork type, using the values of C = 200 pF, R = 100 megohms, and super-gain bipolar transistors having hFE = 5000. While mass fabrication of capacitors in the 200 pF range is feasible, the values of transistor hFE and resistor R profitably attainable for mass fabrication under ordinary bipolar techniques are approximately 1 megohm for resistor R and about 1000 for transistor hFE. Understandably, therefore, the above-mentioned prior art presents a great many problems for mass fabrication that an extremely low yield is obtained and the circuit cannot be put into practical use.
We have, using new fabricating and circuit technologies, previously disclosed a method of fabricating an electronic circuit (shown in FIG. 3) which can be converted to a monolithic integrated circuit device (U.S. Pat. No. 3,905,188). The circuit is divided into three major blocks:
1. a first block, comprising capacitor C.sub.1, resistors R.sub.1 - R.sub.4, and transistors Q.sub.1 - Q.sub.3, which is an amplitude control component of the oscillator and which processes signals from the coil Ls portion of a mechanical-electrical transducer by differentiation with capacitor C.sub.1 ;
2. a second block, comprising resistors R.sub.5 - R.sub.8 and transistors Q.sub.4 - Q.sub.11, which is an amplifier for amplifying signals from the transducer Ls received through capacitor C.sub.2 ; and
3. a third block, comprising resistor R.sub.9 and capacitor C.sub.2, which is a time constant component to control the resonant frequency of the circuit.
The maximum values employed in this circuit are a resistance of 500 megohms, a capacitance of 2000 pF, and an npn transistor having an hFE in the range of 500. A novel feature of the above invention when compared with conventional bipolar circuits resides in the resistor and capacitor components. The resistor, R.sub.9, employs an element (hereinafter referred to as MOS-R) comprising a drain of depletion-type MOS transistor as one terminal, and source, gate and base plate connected in series as the remaining terminal. The capacitor, C.sub.2, has a metal-oxide-alumina-oxide silicon structure (hereinafter referred to as MO'AOS), using said metal layer as one terminal and the silicon layer as the remaining terminal. Although a similar capacitance can be obtained from a common MOS formula for the capacitor, i.e., metal-oxide-silicon, the aforementioned MO'AOS formula is more advantageous for fabrication when combined with MOS-R. Even with this fabricating technique, the circuit is still complex as can be seen in FIG. 3.
Consider the operating principle of the device shown in FIG. 2. Normally, transistor Qo will be biased to be approximately at the cut-off point. When a switch is closed and a step voltage from the circuit voltage supply is applied to the node of coil Ls and the RC tank circuit, the voltage across the RC tank circuit will ring about the cut-off point of transistor Qo. The collector current will therefore have an impulse frequency as determined by the RC tank circuit. If the resonant frequency of the collector current or RC tank circuit is matched to the resonant frequency of a mechanical oscillator or a tuning fork, a sympathetic voltage and current may be set up in sensing coil Ls. The voltage induced in coil Ls will reinforce the resonant ringing of the RC tank circuit and a self-sustaining oscillation will be set up. Thus, the time constant of the RC tank circuit must be chosen to match the resonant frequency of the oscillator means. If RC time constant is shorter than the oscillator's resonant value, the base bias will oscillate too fast to drive the collector current at the oscillator's resonant frequency. The oscillator will accordingly be driven at a frequency less than its resonant frequency. The reverse situation can easily be surmised. As a consequence, excessive power may be consumed or the oscillator means may fail to resonate with sufficient amplitude to drive the clock mechanism.
When used in a timepiece of the balance wheel type, the FIG. 2 circuit having ordinary transistor hFE values requires the values of R = 10 megohms, C = 0.47 .mu.F, and produces a time constant of 4.7 second; whereas the FIG. 3 circuit without the first circuit block, the amplitude control component, requires the values of R.sub.9 = 500 megohms, C.sub.2 = 200 pF, and has a time constant of 0.1 second. Thus, the prior art circuit illustrated in FIG. 3, having only the second and third blocks, the amplifier and time constant components, produces resonant oscillation having a shorter period and a higher frequency than the FIG. 2 circuit, making the FIG. 3 circuit unsuitable as a drive circuit for a balance wheel type timepiece. The time constant R.sub.9 C.sub.2 needs to be an order of magnitude higher. However, the fabrication of a capacitor and resistor having a RC product in the range of 1 to 10 seconds is entirely impractical according to present integrated circuit mass production techniques. However, our prior art circuit, illustrated in FIG. 3, has a low enough effective resonant frequency so as to be successfully coupled to balance wheel oscillators.
A major drawback of the FIG. 3 circuit lies in the amplitude control component, the first block, whose function is to lengthen the time constant of the second and third circuit blocks, i.e. the amplifier and time constant components, in order to make the circuit usable in balance wheel timepieces. The design criteria and tolerances of the circuit portion denoted as the first block in FIG. 3 are very critical and the overall performance of the circuit is extremely sensitive to small variations. Without exception even the most careful mass production processes have obtained very small yields of this circuit and small improvements in yields are difficult to achieve and are obtained only at high cost.
Therefore, what is needed is an integrated circuit amplifier and oscillator which is capable of being combined with a mechanical oscillator, and which is adapted to economical and practical mass production techniques without requiring large resistance, capacitance or transistor hFE values.